Field of the Invention
The present invention relates to a birefringent filter unit.
Description of the Related Art
Optical imaging apparatuses are being actively studied in the medical field, which irradiate an organism with light from a light source such as a laser so as to image information relating to the interior of the organism that is obtained based on the light entering the organism. Photoacoustic Tomography (PAT) is one such optical imaging technique. In PAT, an organism is irradiated with pulsed light generated by a light source, and an acoustic wave generated in the organism tissue, which has absorbed the energy of the pulsed light having propagated and diffused through the organism, is detected. This phenomenon of the generation of a photoacoustic wave is referred to as a photoacoustic effect, and an acoustic wave resulting from the photoacoustic effect is referred to as a photoacoustic wave. A segment to be examined such as a tumor often has a higher light energy absorptance compared to surrounding tissues and thus absorbs a larger amount of light than the surrounding tissues and instantaneously expands. An acoustic wave detector is used to detect a photoacoustic wave generated during this expansion to obtain a reception signal. The reception signal is mathematically analyzed to allow imaging of the sound pressure distribution, in the object, of the photoacoustic wave resulting from the photoacoustic effect (the resultant image is hereinafter referred to as a photoacoustic image). Based on the photoacoustic image thus obtained, an optical-characteristic distribution (particularly an absorption coefficient distribution) in the organism can be acquired. Such information can also be utilized to quantitatively measure certain substances in the object, such as glucose or hemoglobin contained in the blood. Currently, photoacoustic image apparatuses are being actively studied which use PAT and are intended to image blood vessels in small animals or be applied to diagnoses of breast cancer or the like.
In-vivo substances such as glucose and hemoglobin vary in light absorptance depending on the wavelength of incident light. Therefore, the distribution of in-vivo substances can be accurately measured by irradiating an organism with light having different wavelengths and analyzing resultant differences in absorption coefficient distribution. In general, light with a wavelength of 500 nm to 1,200 nm is used as irradiation light. In particular, when absorption by melanine or water needs to be avoided, near infrared light with a wavelength of 700 nm to 900 nm is used as incident light.
An alexandrite laser and a titanium sapphire laser are wavelength variable lasers having gain bands in the above-described wavelength range. Examples of a wavelength selection method for the wavelength variable laser include a method of rotating a mirror in a laser resonator with a wavelength dispersion element such as a prism or a diffraction grating arranged therein, a method of using a birefringent filter that is arranged in a laser resonator, and a method of utilizing an acoustic optical element. The birefringent filter method uses a member including a number of birefringent plates (thin plates of birefringent optical elements) arranged parallel to one another so as to be mutually spaced apart by spacers or the like. The optical axes of the birefringent plates lying in the respective planes of the plates are arranged in such a particular angular relation as allows a desired wavelength to be selected. When a wavelength is selected, the whole birefringent filter is rotated with the angular relation maintained and with surfaces of the birefringent plates kept parallel to one another (Japanese Patent Application Laid-open No. 2014-150243).
The number of birefringent plates that is suitable for improving a wavelength selection characteristic is, for example, three (S. M. Kobtsev et al., “Application of birefringent filters in continuous-wave tunable lasers: a review”, Optics and Spectroscopy 73(1), 114-123, July 1992).